Define a molecule. How many molecules are present in

(a) 9 gram of water

(b) 17 gram of ammonia

A molecule is defined as a group of two or more atoms combined through a chemical bond. For example: ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{NH}}_3$ etc.

(a) Finding the number of molecules of water in 9 g of water

The molar mass of water = $18{\mathrm{gmol}}^{-1}$

Given the mass of water = $9\mathrm{g}$

$$\begin{gathered} Numberofmoles=\frac{\mathrm{Given}\mathrm{mass}\mathrm{of}\mathrm{molecule}}{\mathrm{Molecular}\mathrm{mass}\mathrm{of}\mathrm{water}} \\ Numberofmoles\mathrm{water}=\frac{9}{18}=0.5mole \end{gathered}$$

Hence the number of moles of water is 0.5.

1 mole of water = $6.022\times {10}^{23}molecules$

0.5 mole of water = $0.5\times 6.022\times {10}^{23}=3.011\times {10}^{23}molecules$

Hence, 9 g of water contains $3.011\times {10}^{23}$ molecules.

(b) Finding the number of molecules of ammonia in 17 g ammonia

The molar mass of ammonia = $17{\mathrm{gmol}}^{-1}$

Given the mass of water = $17\mathrm{g}$

$$\begin{gathered} Numberofmoles=\frac{\mathrm{Given}\mathrm{mass}\mathrm{of}\mathrm{molecule}}{\mathrm{Molecular}\mathrm{mass}\mathrm{of}\mathrm{water}} \\ Numberofmoles\mathrm{ammonia}=\frac{17}{17}=1mole \end{gathered}$$

Hence the number of mole of ammonia is 1.

1 mole of ammonia = $6.022\times {10}^{23}molecules$

Hence, 17 g of ammonia contains $6.022\times {10}^{23}$ molecules.